The present device relates to instruments and systems for detecting apnea and respiratory arrest, and to systems wherein such a detector is used in conjunction with an alarm or a prompt to summon help, jar an individual back to a normal breath cycle, or otherwise respond to or treat the detected condition.
Breathing is normally characterized by a more or less regular rhythm of inhaling and exhaling. Apnea, or cessation of respiratory airflow, is a condition in which pauses of greater or lesser duration occur. These pauses may be of such length as to appear to be, or to actually amount to complete cessation of breathing. Apnea may be caused by a number of different mechanisms including obstructive episodes in the upper airway, by neurologic or disease-mediated lack of diaphragmatic motion, or by a combination of these factors. Although poorly understood, sleep apnea has been associated with sudden infant death syndrome (SIDS) which kills hundreds of infants annually. In addition, children are particularly vulnerable to apnea after general anesthesia, and both children and adults receiving epidural narcotics and local anesthetics are at increased risk of apnea and respiratory arrest.
For the latter group of people, a number of monitoring devices have been developed or proposed. These include respiration monitors which detect the carbon dioxide level in air exhaled by the patient, transcutaneous oxygen monitors or pulse oximetry instruments, as well as diverse instruments which sense motion of the abdomen. Among the latter may be counted an elastic strain-sensing belt that may be worn over clothing, and infrared movement detectors which have been used in research environments. In addition, acoustic detectors configured to detect the sound of breathing have been used together with specially programmed microprocessors to determine when cessation of breath or lapses in the breath rate occur, and to trigger appropriate hospital alarms. U.S. Pat. No. 5,551,418 shows a device for coordinating airway pressure with respiration of the patient, and U.S. Pat. No. 5,546,933 shows a system wherein a positive pressure breathing source is controlled based on detecting flow limitation of an inhalation wave form.
A significant drawback of the strain-sensing belt technique is that it typically requires that the belt be affixed to the patient. The corresponding wires that couple the belt with the detection unit markedly increase the risk that the subject will become entangled in the wires, which is undesirable, especially with small children. Moreover, if the subject is experiencing unrestful sleep which is accompanied by tossing and turning, the wires may detach from the unit or the belt, thereby creating false alarms.
A drawback of the remaining techniques is that movement of the subject interferes with the monitoring of the subject. Consequently, the system may generate false alarms, which are bothersome to the subject and to the attending clinicians in a hospital environment.
In addition to the foregoing approaches specifically related to apnea detection, there exist a number of other techniques for the measurement of motion which have not seen application to apnea detection. Among these techniques should be mentioned Lidar, a radar-like ranging technique which determines the velocity of a moving surface by projecting a short burst of light at an object and determining the round-trip transit time of light reflected off the object and which is returned to a photodetector. This technique has been applied with appropriate timing circuits and detection devices, to determine both the distance and velocity (obtained from the change in distance at two times) for moving objects such as automobiles, for which purpose the technology is similar to radar speed detectors, but is useful generally at lesser ranges. Similarly, reflected light figures in laser Doppler measurements, which rely on the reflection of incident light off of moving surfaces, such as particles in a flow stream. In this case, the light beam is a coherent beam which is preferably continuous, and the motion information is detected by mixing a return beam with a reference beam on a photo-detector and analyzing the detected signal by a Fourier transformation to determine the frequency components present in the return beat signals present in the mixed beam. This measurement technique, laser Doppler velocimetry, has been applied for example to determine the flow velocity of blood in retinal capillaries and in the skin, to measure flow velocity by introducing aerosol particles as reflectors in an airstream, and to measure fluid flow in other situations. The Doppler measurement technique is well adapted to detection of particle motion at velocities well below one meter per second. To applicant's knowledge, however, neither up these techniques have been applied to any form of patient breath monitoring.
In general, it is desirable that a sleep apnea monitor be non-invasive, and, if attached to the patient, that it employ sensing units which are not subject to entanglement or constriction, and further pose no hazard of laceration, suffocation, or electrocution. In addition, such a sensor should be of general applicability so that for example the presence of a particular incision or operative trauma does not preclude use of the monitor. There remains a need for such a general apnea monitor.
Accordingly, it would be desirable to provide an apnea detection and treatment system of general applicability which is safe, non-invasive, and effective.